Along with an increase of the depth of the wells for oil fields and gas fields, higher strength has been in demand for oil well pipes used in oil wells or gas wells. Instead of 80 ksi class (yield stress (YS) of 551 to 654 MPa) or 95 ksi class (YS of 654 to 758 MPa) of oil well pipes generally used so far, oil well pipes of 110 ksi class (YS of 758 to 861 MPa) have been used frequently in recent years.
On the other hand, recently developed deep wells often contain corrosive hydrogen sulfide. Under such circumstances, high strength steels cause hydrogen brittleness which is referred to as sulfide stress cracking (hereinafter referred to as “SSC”), which sometimes results in the fracture of the oil well pipes. Accordingly, the most significant subject in high strength oil well pipes is to overcome SSC.
One method of improving the SSC resistance in oil well pipes of the 95 to 110 ksi class is to purify steels highly or to refine the crystal grains of the micro-structure. For example, Patent Document 1 proposes a method of improving the SSC resistance by reducing the impurity elements such as Mn and P. Patent Document 2 describes a method of improving the SSC resistance by refining crystal grains by quenching twice.
Further, studies on high strength oil well pipes, such as of 125 ksi class (YS of 861 to 965 MPa) have been started in recent years. Since SSC is more likely to occur in the higher strength steels, a further improvement in the material design for oil well pipes of 125 ksi class has been in demand than for prior oil well pipes of the 95 ksi class (YS of 654 to 758 MPa) or the 110 ksi class (YS of 758 to 861 MPa). Patent Document 3 proposes a method for obtaining steel materials of the 125 ksi class which is excellent in the SSC resistance that the crystal grains of the micro-structure is refined by a heat treatment using an induction heating. Patent Document 4 proposes a method for manufacturing a steel pipe of the 110 to 125 ksi class and in the 140 ksi class (YS of 965 to 1068 MPa) which is excellent in the SSC resistance, by increasing the hardenability and the tempering temperature with a direct quenching method. Patent Document 5 proposes a technique of obtaining a low alloy steel of the 110 to 140 ksi class with excellent SSC resistance by optimizing the alloy composition. Patent Document 6, Patent Document 7, and Patent Document 8 propose methods for improving the SSC resistance of a low alloy steel used for oil wells of the 110 to 140 ksi class by controlling the forms of carbides. Further, Patent Document 9 proposes a technique of retarding the occurrence of SSC of the 110 to 125 ksi class steel materials by precipitating a large amount of fine Vanadium carbides.    [Patent Document 1] Japanese Patent Unexamined Publication No. S 62-253720    [Patent Document 2] Japanese Patent Unexamined Publication No. S 59-232220    [Patent Document 3] Japanese Patent Unexamined Publication No. H6-322478    [Patent Document 4] Japanese Patent Unexamined Publication No. H8-311551    [Patent Document 5] Japanese Patent Unexamined Publication No. H11-335731    [Patent Document 6] Japanese Patent Unexamined Publication No. 2000-178682    [Patent Document 7] Japanese Patent Unexamined Publication No. 2000-256783    [Patent Document 8] Japanese Patent Unexamined Publication No. 2000-297344    [Patent Document 9] Japanese Patent Unexamined Publication No. 2000-119798